On a surface of a solid substrate or a thin film, an oxide film, where atoms constituting the solid substrate are combined with atmospheric oxygen, or a film, where the atoms are terminated with hydrogen, is generally formed. The film is extremely thin and commonly not thicker than 1 nm. Conventionally, impurities have been physically introduced from above a film such as an oxide film by using a means such as ion implantation. In a word, energy is given to ions, which become impurities, by using an electric field or the like, and the impurities are introduced inside the solid substrate by irradiating the ions to the surface thereof.
Recently, according to miniaturization of devices, a technology for forming a shallow junction has been required. A low-energy ion implantation technique is considered as the conventional technology for forming the shallow junction. The low-energy ion implantation technique is a method for pulling ions out of an ion source with a certain high level of a voltage and decelerating them at a latter stage. This method has been devised for keeping a beam current value with a certain high level and implanting with low-energy. As a result of these devices, formation of a shallow impurity layer of approximately several 10 nm becomes possible, so that it is industrially adapted to a manufacturing process of a semiconductor.
A plasma-doping technique is considered as a notable technique in these years for forming a shallower junction. The plasma-doping technique is a technique for introducing impurities into a surface of an object to be processed (e.g., semiconductor substrate) by contacting plasma including desired particles with the surface of the object to be processed. Because plasma has low-energy of several 100V at the highest, it is suitable for forming a shallow junction, so that experiments for forming shallow junctions of over 10 nm to several 10 nm have been reported.
Further, current experiment achieving the shallowest P-type junction is disclosed in “Technical Digest of Symposium on VLSI Technology, Honolulu, p. 110 (2000)”. This describes a depth of a junction of 7 nm.
Still further, a vapor-phase doping method using a gas source is proposed in “(1) International Workshop on Junction Technology (IWJT), p. 19 (2000)”, “(2) J. Vac. Sci. Technol. A16, p. 1, (1998)”, “(3) Silicon Technology No. 39 18th Jun. 2002” or the like. This is a method capable of forming an impurity diffusion layer of P-type or N-type by heating a semiconductor substrate at a hydrogen atmosphere with an ordinary pressure and supplying B2H6 or PH3. Hydrogen carrier gas removes a natural oxide film on silicon and keeps its surface clean, thereby preventing surface segregation of impurities such as boron.
Generally, a temperature of not lower than 600° C. is needed to decompose gas. For example, “Silicon Technology No. 39 18th Jun. 2002” discloses as an experimental result that a shallow junction of high concentration is formed by heating a semiconductor substrate at 900° C. and supplying B2H6 gas of 1 ppm for 40 seconds. According to this experimental result, a depth that boron concentration becomes 1×1018 cm−3 is defined as a depth of a junction, and the depth of the junction is approximately 7 nm which is the same level as that described above.
Yet further, “International Workshop on Junction Technology (IWJT), p. 39-40 (2002)” discloses a technology that the vapor-phase doping methods are executed at room temperature. These are methods that when material is introduced into a solid substrate where a film such as an oxide adheres to its surface, desired particles are stuck or introduced after removing the film such as the oxide. According to the report, a depth of an impurity-introducing layer is 3-4 nm.
As discussed above, by using the plasma-doping technique or the low-energy ion implantation technique, the experiments for forming shallow junctions of over 10 nm to several 10 nm have been recently reported. The current experiment achieving the shallowest P-type junction forms a shallow impurity layer of approximately 7 nm. However, according to progress to further miniaturization of devices, a method for forming shallower impurity layers more simply with low resistance is required.
As a technology for meeting the need mentioned above, because the plasma-doping technique can introduce particles into a semiconductor substrate with small accelerating energy, the plasma-doping technique can form introducing layers shallower than the ion implantation technique. However, though it is small energy, it has accelerating energy, so that there is a limit to form shallower.
In addition, the plasma-doping is known that a radical is supplied to a substrate as dopant. Because a radical does not have an electric charge, it is not accelerated and struck into the substrate. However, it is thought that because it is active, it reacts to a surface of the substrate and is introduced into the substrate. The vapor-phase doping method using a gas source is a technology that an impurity-diffusion layer is formed by supplying dopant, which does not have accelerating energy, into the substrate and reacting its surface. These are positioned as a technology exceeding a limit of a method for irradiating ions having energy onto the substrate.
However, as mentioned above, because the vapor-phase doping method using a gas source decomposes gas, a temperature of not lower than 600° C. has been generally needed. Photoresist can not be used as mask material at such a high temperature. Therefore, SiO2 or the like is needed to be formed and patterned by using a CVD method or the like, thereby increasing processes for forming transistors.
Furthermore, in a case where dopant, which does not have accelerating energy, such as radical or gas molecule in the plasma-doping technique or the vapor-phase doping method, or dopant having extremely small accelerating energy is introduced into the substrate, it is difficult to form an impurity layer of high concentration for a short time.
According to the vapor-phase doping method adapting a method that desired particles are stuck or introduced after removing the film such as the oxide, an impurity layer of high concentration can be formed at room temperature. However, a method for controlling dose amount has not been proposed.
Conventionally, a method for ion-implanting germanium or silicon is known as a technology for making crystal silicon of the semiconductor substrate amorphous. A process for ion-implanting germanium or silicon into a silicon substrate and making its surface amorphous, then ion-implanting impurities such as boron, and then annealing is widely used. The following advantages of making amorphous before ion-implanting impurities are known: 1) Small impurities such as boron are difficult to be introduced deeply in ion-implanting; 2) Impurities can be activated efficiently in annealing because amorphous silicon has a higher absorption coefficient of light than crystal silicon. However, amorphism by using ion-implantimg does not have enough efficiency for forming a shallow amorphous layer.